WDM technology has provided a cost-effective solution to fiber exhaust in communications networks by increasing the data throughput of the network without requiring the installation of new fiber. In a WDM system each of several input signals enter a WDM node or network element and is assigned or converted to a specific wavelength, typically, in the 1550 nanometer (nm) band. After wavelength conversion each individual signal wavelength or channel is then multiplexed by wavelength division multiplexing and transmitted onto the same fiber. In order for WDM technology to be truly viable as a network solution, WDM systems must also be able to survive faults that occur in any network. The issue of network survivability takes on additional import in WDM systems since the loss of a fiber could be catastrophic and costly given the enormous amount of customer data, e.g., multigigabit data, a WDM system transports on a single fiber.
In response to concerns regarding WDM network survivability, self-healing WDM ring and point-to-point diverse protection architectures have been proposed. A self-healing ring is a network architecture that connects the nodes in a physical ring topology with bandwidth sharing and self-healing capabilities to overcome failures in the network. For the purposes of this description each node in a ring is connected to another node via fiber. If a fiber cut or other fault, e.g., node failure, occurs, then the ring automatically switches to a standby fiber and, in some cases, standby electronics. Point-to-point diverse protection systems similarly protect the network from fiber cable cuts by automatically switching the customer data to a standby fiber routed along a different path. In either case automatic protection switching may be done optically, i.e., by switching the received optical signal to a standby fiber, or electrically, i.e., by switching the electrical representation of the received optical signal. Automatic protection switching in WDM networks promises considerable cost savings relative to pure Synchronous Optical NETworks (SONET) protection. However, before automatic protection switching can be employed in WDM systems some fundamental issues must be addressed.
One such fundamental issue for WDM systems is the detection of fiber cuts in optically amplified links. The detection of a fiber cut or a loss of signal has proven to be a difficult issue in WDM systems because the links between the nodes are usually amplified optically by Erbium Doped Fiber Amplifiers (EDFAs). Typically, in each WDM node the signal is amplified by an EDFA after multiplexing and before transmission onto the network fiber facility or link. Similarly, after reception, in each WDM node the signal is again amplified by another EDFA prior to demultiplexing. Depending on the distance between a transmitter and receiver, one or several additional EDFAs may also be placed at specific points along the fiber path. As the distance and number of amplifiers between the fiber cut and the optical monitor or receiver increases, amplified spontaneous emission from the EDFAs grows with each EDFA in the optical path. Specifically, when there is no optical input signal in a saturated EDFA, the amplified spontaneous emission may increase enough after several EDFAs so that a fiber cut could go undetected. In fact, because of amplified spontaneous emission, measurements of total optical power or even of optical power within a spectral band are insufficient for measuring certain fiber cuts.
Detection of total optical power can fail to detect certain fiber cuts depending on the location of the fiber cut relative to the EDFAs and the detection threshold. On some fiber links or spans there are no EDFAs beyond those in the nodes or network elements, while on others there may be more than one EDFA. FIG. 1 depicts a prior art working fiber/protection fiber pair in a WDM ring including network add-drop elements 120 having protection switches 121 and 122. Specifically, as exemplified in FIG. 1, on a link 110 there are four EDFAs 199 in both the counterclockwise and clockwise directions between two of the add-drop elements 120 (note here that although FIG. 1 shows a ring this discussion also pertains to point-to-point architectures). A fiber cut occurring on sublink 111 could easily be detected at a monitor point 150.sub.1 because the total optical power at the monitor point 150 drops to zero. However, for more remote fiber cuts, such as those occurring at sublinks 112, 113, 114 and 115, the amplified spontaneous emission provided by intervening EDFAs 199 provides optical power to monitor point 150.sub.1.
The relationship between the power detected at the monitor point 150.sub.1 in relation to the number of intervening EDFAs 199 is depicted in FIG. 2. FIG. 2 is a wavelength domain simulation that illustrates the problems with fiber cuts. The simulation assumes specific EDFA characteristics and spacing. Although results for other EDFA designs may differ quantitatively, the qualitative features shown in FIG. 2 will be similar. As FIG. 2 shows, without a fiber cut the total optical power level 201 at the monitor point 150.sub.1 was approximately 18 dBm. If a fiber cut occurred on sublink 112, i.e., with a single EDFA 199.sub.1 before the monitor point 150.sub.1, the total optical power 202 detected at point 150.sub.1 would drop to approximately 4 dBm after 0.5 millisecond (ms). On the other hand, where there were two or more EDFAs between point 150.sub.1 and the fiber cut, i.e., a fiber cut at sublink 113, 114 or 115, the total optical power returned to within 2 dB of the total optical power when the fiber was intact. In fact, when there were either three or four EDFAs between point 150.sub.1 and the fiber cut, the total optical power 204 or 205 never varied more than 4 dB and returned to the total optical power level 201 within less than 0.5 ms. As seen by power level 203, with two EDFAs, the power level also returned almost to the power level 201.
Measurements made on our testbed have confirmed the results depicted in FIG. 2. Based on our simulations and testbed measurements we have drawn the following conclusions with respect to simply monitoring the optical power to detect a fiber cut in a WDM system: when no EDFAs lie between the fiber cut and the monitor point, fiber cuts can be correctly identified; if one EDFA was between the fiber cut and monitor, correct identification of the fiber cut could not be accomplished without careful selection of the detection threshold used to detect a fiber cut; and when more than two EDFAs were located between the monitor point and the fiber cut, a threshold could not be established which would allow for detection of the fiber cut.
We have also investigated and found unsatisfactory monitoring the power within a narrower spectral band to detect fiber cuts at monitor point 150 in lieu of detecting the total optical power in the fiber. In this regard, we have inserted an additional marker wavelength into the fiber at the output of a network element. We found that if the power in the marker was high enough, simple detection of the marker is sufficient to indicate a fiber cut. However, high power at the marker wavelength results in lower EDFA gain for the signal wavelengths and is therefore undesirable. On the other hand, if the marker is at a power level comparable to the signal wavelengths, as shown in FIG. 3, then the marker allows detection of the change from the normal power level 301 to the lower power level 302 for a cut with only one EDFA before the monitor; but the marker alone will not give the contrast required to detect a fiber cut after more than two EDFAs, as shown by power levels 303, 304, and 305. Although we found that a contrast of 10 dB was possible if a very narrowband filter (filter width less than 0.2 nm) was used to generate the spectral band, such a filter places unrealistic demands on marker wavelength filtering. However, note that the width of the narrowband filter is dependent on the test setup.
Other methods are known in the art. One such method has been described by J. L. Zyskind, in U.S. Pat. No. 6,008,915, entitled "Method of Identifying Faults in WDM Optical Networks". In his method Zyskind, uses an additional laser to insert an additional monitoring channel in the WDM system fiber along with the signal channels. The power in the monitoring channel and the amplified spontaneous emission by the EDFAs employed along the fiber path are then monitored and compared to detect faults. That is, a power change in the same direction on the monitoring channel and the amplified spontaneous emission, e.g., both increase or decrease, is interpreted as signal channels being either dropped or added. On the other hand, a power change on the monitoring channel and the amplified spontaneous emission in the opposite direction is interpreted as an overall loss indicative of a fault.
The Zyskind, method requires additional components including a monitoring laser, couplers, and narrowband filters in order to be implemented. More importantly, as the number of channels are added or dropped the power level of the monitoring channel and the amplified spontaneous emission change thereby changing the threshold level for detecting faults. Zyskind's method also requires a fairly sophisticated detector that would be required to keep track of five different cases for upstream loss and signal channels that could occur. This method, therefore, would probably require decision making software.
In their paper entitled "A Novel In-Service Surveillance Scheme for Optically Amplified Transmission Systems" (published in IEEE Photonics Technology Letters, Vol. 9, No. 11, November 1997) Chan, Chun-Kit, et. al., described another prior art approach for detecting faults in WDM systems. Chan, et. al., utilize the nonflat amplified spontaneous emission spectra of the EDFAs as the light source for monitoring the fiber channel for fault. By the Chan, et. al., method fiber Bragg gratings are placed close to the input end of each EDFA, except the first EDFA after the transmitter, along the fiber path. Each fiber Bragg grating then filters a distinct wavelength within the unused spontaneous emission spectra. Each filtered wavelength is assigned to each amplifier immediately preceding a fiber Bragg grating. Because the fiber Bragg grating operates as notch filter, a power loss occurring upstream of the fiber Bragg grating results in a spectral pulse at that fiber Bragg grating distinct wavelength. By this method, fiber cuts can be localized to the fiber span between any two amplifiers. While this method does not require the use of additional lasers, it does require fiber Bragg gratings as additional components. This method would also require sophisticated spectral monitoring. This method also may not be able to detect fiber breaks that occur between a fiber Bragg grating and the input of its assigned amplifier, nor will partial failures of certain amplifiers be detectable.
All the above approaches either require additional components or are not able to detect all fiber cuts, regardless of the location of the fiber cut relative to an amplifier or a number of amplifiers.